The topical delivery of an active agent, such as a drug, from a given vehicle is a multistep process which involves; 1) the dissolution, diffusion and/or release of the agent from the vehicle; 2) the transport of the agent to the absorption site; 3) the permeation of the agent through the limiting barrier at the target site; and eventually 4) the elimination of the agent via blood circulation. In order for an active agent to be effectively transported into the epidermal absorption site, the agent must be present at the skin/vehicle interface, be soluble in the available surrounding fluids, and be capable of diffusing down the hair follicle or partitioning into the stratum corneum.
It is well known in the art that the skin presents a formidable barrier to the penetration of active agents due to the presence of the stratum corneum, which is the outermost layer of the skin composed of close packed dead keratinized cells. Thus, the ability of a substance, which is applied to the skin surface, to penetrate through the skin is inversely related to the thickness of the stratum corneum layer.
One known method for enabling the penetration of active agents through the skin is via topical administration. One shortcoming associated with this method is that some active agents, in particular those having a relatively high molecular weight, are not easily absorbed by the skin.
Many methods for enhancing the local penetration of pharmacologically active agents are known in the art. In one approach, surface active agents are included in the pharmacologically active agent-containing formulation. However, this addition of surface active agents results in only a slight to moderate enhancement of permeability at the expense of damaging the barrier skin tissue. See, e.g, Zatz, J. L., Modification of Skin Permeation by Surface-Active Agents in Skin Permeation Fundamentals and Application, 149-162 (1993); and Barry, B. W., "Properties That Influence Percutaneous Absorption in Dermatological Formulations," Percutaneous Absorption, 127-233 (1983). (Addition of "penetration enhancers" increases the permeability of the stratum corneum by disrupting the lipid organization therein.)
Traditionally, the prime pathway for the topical delivery of active agents across the skin was thought to be through intercellular routes and transcellular routes of the stratum corneum. However, alternative means such as via appenageal transport, i.e., follicular transport, is gaining more acceptance in the scientific community. See, e.g., Schaefer, H., et al. Follicular Penetration in Prediction of Percutaneous Penetration: Methods, Measurements, and Modeling, 163-173 (1990)(optimum size for uptake of micro-beads by the skin appendages is between 5-7 .mu.m). Due to the comparatively lower barrier resistance of the follicles, follicular transport has particular relevance for enabling the delivery of active agents having low diffusion constants, varying solubility characteristics, and/or varying size characteristics.
One of the diseases associated with the hair follicle is androgenetic alopecia ("AGA"), which is the most common cause of hair loss in both men and women and is identifiable by the loss of hair over the vertex of the scalp. Commonly known treatments for AGA include hair follicle transplants, topical therapies, and orally prescribed anti-androgens. See, e.g., J. L. Roberts, "Androgenetic Alopecia in Men and Women: An Overview of Cause and Treatment," 9(6) Dermatology Nursing 379-386 (1997).
Minoxidil is a drug that is often prescribed for the treatment of AGA. Disadvantageously, clinical trials have shown that the topical application of a 2% minoxidil solution to patients experiencing hair loss results stimulated dense hair regrowth in only less than about 5% of the patients and moderate hair regrowth in only about 30% of the patients. See e.g., E. A. Olsen, et al., "Topical Minoxidil in Early Male Pattem Baldness," 13, J. Amer. Acad. Derm. 185-192 (1985); and J. Roberts, "Androgenetic Alopecia: Treatment With Topical Minoxidil," 16(3) J. Amer. Acad. Derm. 705-710 (1987). Moreover, treatments with topical solutions of minoxidil require multiple daily applications, which is not only inconvenient but also expensive.
One known method for the topical administration of minoxidil is via a mousse formulation containing a cellulose derivative film former as disclosed in PCT Patent Application WO 88/01863. Disadvantageously, such cellulose derivatives hinder the diffusion of the minoxidil from the vehicle, which thereby leads to decreased efficacy of the minoxidil.
Various delivery systems are also known in the art for topically delivering anti-acne agents, anti-aging agents, and depigmentation agents to the epidermis and/or dermis. In one method, high levels of alcohol are used to faciliate the delivery of the agent. However, such alcohols disadvantageously tend to extract lipids from the skin surface, which leads to excessively dry skin and irritation. See, e.g. Zatz, J. L. Modification of Skin Permeation by Solvents in Skin Permeation Fundamentals and Application, 146-148 (1993).
It is therefore an object of this invention to provide improved compositions for enabling the topical delivery of pharmacologically active substances without significant adverse side effects on the skin or body membranes. It is also a further object to provide improved compositions for enabling the topical delivery of difficult-to-absorb agents for localized action. It is a further objective to provide an economic, effective topical treatment for hair loss that is convenient to apply. It is yet a further objective to provide improved compositions for enabling the topical delivery of anti-dandruff, depilatory, anti-acne, anti-aging, and depigmentation active agents into the skin.